376 MESSRS. A. V. JENNINGS AND G. J. "WILLIAMS 



frap:raonts of slate with distinct cleavage, and containing Lingulce, 

 indicating that older aqueous rocks had already undergone pressure 

 and alteration before the later Arenig eruptions. In the Middle 

 Agglomerate about Llyn Trwstyllog, where the base is bluish and 

 full of i)orphyritic felspars, there occur indefinite slaty patches 

 which seem on the other hand to have been ejected in a soft condition, 

 as in the tuffs on Cader Idris. 



Of the rocks contained in the agglomerates we have made no 

 d(.^tailed study ; but they are for the most part slaty and schistose 

 fragments mixed with rounded pebbles of fine-grained " felstone." 



It will be noted that the igneous rocks are of acid composition 

 throughout, and there is not that transition from basic eruptions 

 below to acid lavas above which is so marked on Cader Idris. 



The "Middle-Slate bed runs under the lake of Trwstyllog and 

 up towards Moelwyu Bach ; its course across the Cwm being 

 traceable by the succession of trial-levels, and the low ground it 

 forms between the harder rocks above and below. Northwards, it 

 forms the strip of marshy land lying above the Lower Agglomerate, 

 and can be traced along to the AVrysgan Quarry on the line of 

 Section 2 (see p. 371). In spite of the high angle of dip, the 

 quarry is still worked, though the quality of the slate is not so good 

 as that of the Llandeilo beds, owing to hardness and coarseness 

 below, and the presence of sandy bands in the up})er part. 



The Upper-Slate bed runs round Cwm Trwstyllog, above the 

 lake, and has been worked on the south-west side of the hollow. 

 Traced northwards it seems to descend towards the Middle-Slate 

 bed, owing to the thinning of the Middle Agglomerate. This does 

 not disappear entirely as represented in the Survey map and memoir. 

 A short distance beyond the marshy ground, where it seems lost, the 

 Upper-Slate bed reappears and can be traced along under Poel 

 Hvdd, where it is represented on our second Section above the 

 Wrysgan Quarry. Between the quarry and Cwm Orthin, the 

 Middle Agglomerate thins rapidly, resultins: in the approximation 

 and coalescence of the Upper- and Middle-Slate beds. 



(d) The Llandeilo Series, — Lying on the Upper-Agglomerate 

 series come the slates which we regard as of Llandeilo age, dipping 

 westward toward Cynicht. They form the top of Moelwyn Mawr 

 and Foel Rydd, but have been removed from Craig Nyth-y-Gigfran. 



Immediately above the Upper Agglomerate at Llyn Bowydd occur 

 graptolites identified by Prof. Lapworth as Climaco(/raj)tus Scharen- 

 hergi and Dirl ymograptus Murchisonii^ var. hifidus (Hall), which are 

 typical Llandeilo forms. Aj)art from other evidence, Prof. Lapworth, 

 who has kindly examined the specimens, regards the Llyn-Bowydd 

 fossils as belonging " to the highest zone of the Skiddaw Slates or 

 lowest beds of the Llandeilo." 



At Ty Obry, in strata certainly not higher in the series, are found 

 Climacogra])tus celcitus, Lapw., and Diplogr^aptus dentatus, Brongn. 



